Power amplifiers are used in radio transmitters installed at base stations of a wireless communication network, such as in evolved node B (eNB) base stations in long term evolution (LTE) wireless communication networks. These power amplifiers require DC biasing for proper performance at radio frequencies (RF). For example, a transistor technology popularly used in power amplifiers is laterally diffused metal oxide semiconductor field effect transistors (LDMOSFET). Other technologies include Gallium Nitride (GaN). In either of these two technologies, as well as other technologies, various DC biasing techniques are used.
Using LDMOS as an example, the LDMOS transistors are designed for high frequency and high power operation. However, these devices are limited by significant drifts of quiescent current (Idq) at a fixed gate bias voltage (Vgs) as temperature varies, due to the charge build-up in the drain-gate region that is caused by hot carrier injection effects. The quiescent current (Idq) changes proportionally with both the gate bias voltage (Vgs) and temperature. To maintain the maximum output power with high linearity, the Idq should be constant over time across all operating temperatures. To achieve this, the Vgs should be adjusted during operation to compensate for temperature changes.
FIG. 1 shows a block diagram of a known LDMOS bias control circuit 10 using a digital to analog converter (DAC) 12 and a temperature sensor 14 to control a bias voltage at the input 16 of a power amplifier 18. At a fixed gate bias voltage (Vgs), the drain current (ids) drifts as temperature changes. Below a zero crossover point, Ids increases with higher temperature. Above this point Ids increases with lower temperature.
In order to keep Idq constant over the operating temperature range, a micro-controller unit (MCU) 20 measures the temperature changes using the temperature sensor via an analog to digital converter 21 and sets a new bias voltage via the DAC 12. A look-up table 22 provides a gate voltage (Vgs) for a given drain current (Ids) at a given temperature. The Vgs from the lookup table is fed to the DAC 12 which converts the Vgs to an analog DC biasing voltage. Note that the drain current (Ids) is equal to the quiescent drain current (Idq) plus the current induced by the RF signal (Irf) applied to the power amplifier 18. Note also that Ids may be directly controlled by the drain voltage supply 23. The drain voltage ideally remains constant over temperature. The drain current may be measured by an analog to digital converter (ADC) and current sensor 24. In FIG. 1, the dashed lines represent the signal and control lines from and to the MCU 20 used for controlling Vgs, and the solid lines represent the conventional signal and biasing lines of a power amplification system. Further, the feedback receiver 30 and the pre-distortion engine 28, are for conventional pre-distortion functions and the DC blocking components 31 are for isolating the DC biasing voltages.
A known procedure for keeping the drain current (Idq) constant over temperature is as follows:
Pre-store the Vgs vs. temperature data in the look-up table 22;
Measure temperature periodically; and
Control, via the MCU 20, the DAC 12 output voltage for a new Vgs voltage using the look-up table 22.
To ensure proper performance of the power amplifier 18 over temperature, a constant quiescent drain current (Idq) is desired. In the method described above for attempting to keep a constant Idq, temperature is used as an address to select an appropriate gate voltage (Vgs) value to be applied to the input 16 of the power amplifier 18. A problem with the above-described method is that a value of VGS needed to obtain a constant Idq varies from device to device. This variation is so significant that each power amplifier must be calibrated during production to develop a custom look-up table to select Vgs for a given Idq. Average Vgs versus temperature is captured experimentally by measuring four to eight units, often from the same lot, over the operating temperature range. This data is then used for all radios that use the same power amplifier. Normal radio production volumes are in excess of tens of thousands and variations from unit to unit are significant.
A known procedure for generating a Vgs look-up table 22 is as follows:
(a) Select 4 to 8 units already mounted into a power amplifier prototype board and place them into a temperature chamber at a starting temperature of 25 degrees Celsius (C.);
(b) Apply Vgs to each of the units to drive the Idq to a desired level;
(c) Record each Vgs;
(d) Lower the temperature to a desired minimum, (e.g., −40 degrees C.), and repeat steps b and c;
(e) Increase temperature by 5 degrees C. and repeat steps b and c;
(f) Keep repeating step e until the maximum temperature is reached (e.g., 60 degrees C.);
(g) Generate an over-temperature Vgs curve for each transistor;
(h) Normalize all generated curves such that they intercept each other at 25 degrees C.; and
(i) Average all curves to produce a final generic over-temperature curve.
During the radio manufacturing process, each transistor on the power amplifier board is calibrated to select Vgs at 25 degrees C. This value is used in conjunction with a generic Vgs over-temperature curve to generate look-up data that is stored in the look-up table 22. The procedure described above is labor intensive and slow. Further, these steps do not account for changes arising from long term aging of the transistor. Also, the drain voltage 23 applied to the power amplifier 18 may be lowered to achieve greater efficiency when there is a power headroom in the power amplifier 18 or when operating the power amplifier 18 at levels lower than designed for, Lowering the drain voltage 23 reduces the drain current Ids, which in turn requires re-calibration and new values for the look-up table 22. Further, if the Vgs-temperature data of the look-up table 22 is lost in the field, the radio must be returned to the factory for re-calibration.